Dock levelers are utilized to compensate for height differences between the dock platform and the bed of a parked vehicle to permit forklift trucks and personnel to readily move on and off the vehicle during loading and unloading operations. A typical dock leveler has a dockboard or deck pivotally hinged at its back edge for varying the height of the dock leveler in order to compensate for the height differences. An extension plate or lip is typically hinged to the front edge of the deck for spanning the distance between the rear end of the vehicle bed and the outer front edge of the deck. The lip is adapted to move relative to the deck front edge between a pendant stored position, an extended preparatory position, and a lowered operative position. In its operative position, the lip extends from the deck front edge and may be substantially coplanar with the upper surface of the deck. The lip rests upon and is supported by the upper surface of the bed of the parked vehicle. Thus, the extended lip spans the gap which is formed between the rear edge of the vehicle bed and the front edge of the deck.
To move the lip from the pendant stored position to an extended operative position, the deck is normally pivoted upwardly a sufficient amount whereby the lip can swing outwardly to the extended preparatory position without obstruction from the parked vehicle. Once the lip has cleared the parked vehicle, the deck and extended lip are pivoted downwardly as a unit until the extended lip rests upon the vehicle bed in the operative position. After loading or unloading of the parked vehicle has been completed, the vehicle is moved away from the dock permitting the lip to be cycled to assume its pendant stored position.
The dock leveler may also include a safety support system which limits the vertical fall that would occur in the event that the vehicle would prematurely depart the loading dock while a material handling vehicle or other load is present on the dock leveler. Typical safety support systems are hydraulically activated or mechanically actuated.
Hydraulically activated dock levelers are typically equipped with a flow sensing device that closes if and when a predetermined fluid flow rate or cylinder piston velocity is achieved. Such devices allow for normal free floating of the dock leveler to accommodate the vertical height of the vehicle bed, but will lock upon the rapid acceleration that occurs after the vehicle departs the loading dock while a significant weight is present on the dock leveler deck.
Hydraulic safety systems have a disadvantage since they are designed to prevent activation of the safety support system until after the vehicle has completely separated from the dock leveler. Similarly, some previous mechanical systems do not work until after the lip disengages the bed.
Mechanically actuated dock levelers typically have a safety leg assembly which limits the vertical fall that may occur due to the premature departure of the vehicle/trailer and which supports the deck in a horizontal cross traffic position. The safety leg assembly typically has a plurality of legs pivotally mounted to the deck for movement between (1) an active supporting position in which the legs support the deck, and (2) a retracted, inactive, or non-supporting position in which the leg is relatively parallel to the deck. The legs are typically pivoted from the supporting position to the non-supporting position in response to a pull chain mounted on the deck to allow the leveler to service below-dock level vehicles. The legs are typically spring-loaded so that they return to the supporting position upon release of the chain, although some mechanical safety leg systems retain the legs in the non-supporting position whenever the lip is in the extended position.
These mechanical safety leg systems have a certain shortcoming. If the vehicle prematurely departs from the loading dock while a load, such as a forklift or cargo, remains on the deck, the safety legs may not be able to react quickly enough to move to their pendant and active supporting position in time to prevent the rapid descent of the deck to the floor of the pit, causing the load on the deck to spill onto the driveway or potential injury to dock personnel.
Completely manually operated safety legs have a different shortcoming, referred to as "stomp-out" which is caused by the uncontrolled premature obstruction of safety legs when the initial height of the vehicle bed is slightly above the height at which the safety legs engage their stops. When a relatively heavy forklift or other material handling vehicle drives onto the vehicle bed, the height of the vehicle bed is lowered so that the lip follows the descending vehicle bed, forming a relatively large angle between the lip and the deck and making it difficult or impossible for the material handling vehicle to exit the vehicle. The safety legs engage the stops and prevent the deck from descending and maintaining the desired substantially planar orientation with the lip.
Another shortcoming is the safety legs must be manually positioned to a non-supporting position when servicing below-dock level vehicles. Attempts to provide a leg which would automatically assume the retracted or non-supporting position without conscious action by the dock personnel have not been fully satisfactory because the devices resulted in unsafe premature retraction of the legs or have been highly susceptible to malfunction or are difficult and awkward to install and maintain in proper working order. One such attempt is described in U.S. Pat. No. 3,995,342 which issued to Thomas J. Weiner on Dec. 7, 1976. The legs are coupled to the lip so that the legs automatically pivot from the supporting position to the non-supporting position in conjunction with upward movement of the lip from the pendant position toward the extended position. After the lip is extended, the legs can only move to the active supporting position after the lip engages the vehicle bed. If the dock leveler is activated in the absence of a vehicle, the legs will remain in the inactive and non-supporting position, unable to prevent the rapid descent of the dock leveler upon the application of a load on the dock leveler. When the safety legs are in the non-supporting position and the lip is supported by the vehicle bed, they may not be able to react quickly enough to move to their supporting position in time to prevent the rapid descent of the deck to the pit floor if the vehicle prematurely departs from the loading dock while a load, such as a forklift truck or cargo, remains on the deck.
Various safety structures have been proposed in the past which would be responsive to rapid descent of the deck to catch the deck and prevent the descent. Such safety structures, for the most part, are complicated mechanisms and were actuated only after the lip lost contact with the vehicle bed and the deck began to fall, and thus were not entirely effective in restraining the rapid descent of the deck.